This may be an improvement over a text description of the item in question, but it compresses a lot of detail into oblivion. What if, rather than relying on the client to generate the thumbnail, the author generated it? I’m calling this a page placard.

Each page could use the link element to refer to its placard, So for this page, the link might look something like this:

I’m imagining 128 x 96 pixels as a suitable placard size. In the simplest case, the placard would be a static graphic–like a favicon on steroids. So for this blog, it might be something like this:

A more sophisticated approach would be to generate the placard on the fly (and, of course, caching it) using ImageMagick or the like: for a blog, it might be using a branding icon and post title text; for flickr or a photo-gallery app, it might be a thumbnail of the current graphic with a branding icon as a screen-bug.

So a more sophisticated version of the above placard might look like this:

A placard for a photo page on Flickr might look like this:

Whenever a client needs a thumbnail of a page, it looks for the placard link first, and if available, uses that. Letting the author control the appearance of the placard should result in more relevant and immediately legible thumbnails.

This also suggests that even in the absence of an author-defined placard, it would be possible for the client to cobble together something like a placard using, say, the favicon and an element or two from the RSS feed. Heuristics or templates could be used with popular sites to customize how those placards were generated.